1. Field of the Invention
The present invention relates to a process for making silicon carbide. Particularly, the process according to the present invention can be used to make silicon carbide of high purity by low-temperature heat treatment so that energy is significantly saved.
2. Description of Prior Art
Silicon carbide possesses many excellent qualities as high rigidity, wear-resistance, erosion-resistance and is chemically stable. Therefore, it has been widely used as refractory material in the industry and also as a primary material in the making of cutting and grinding tools.
In the present day, a conventional process for mass-production of silicon carbide is the Acheson method which is carried out in solid phase. In the process, a mixture of char and silicon dioxide is placed in batch into an oven and then heated therein with a temperature of 2000.degree. C. After a long time of reaction, .alpha.-type silicon carbide in lumps can be produced. The chemical equation of the reaction is as follows: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO
It is a drawback of the foregoing process that about 8,000 KWH of electrical power is needed to produce a ton of coarse blank of silicon carbide. Energy consumption is therefore very great.
The process for production of .beta.-type silicon carbide uses a powder of high-purity silicon dioxide and metal mixed with a powder of char and then heats the mixture in a protective atmosphere with a temperature in the range 1500.degree. C.-1800.degree. C. The chemical equation for the reaction is as follows: EQU SiO.sub.2 +3C.fwdarw.SiC+2CO or Si+C.fwdarw.SiC
Besides the solid state reaction, the production of .beta.-type silicon carbide can also be carried out by gas phase synthesis in which the reactants are oxidized under high temperatures to form the product.
All of the foregoing processes require great amounts of energy to perform. There exists therefore a process that can be used to produce silicon carbide with low energy consumption.
It has been found that in the reaction of SiO.sub.2 +3C.fwdarw.SiC+2CO, SiO.sub.2 is not only used as the material for the carbonization stage, but also as a silicon source. Through research effort for the present invention, it is found that the reduction capability of the mesophase developed in the heat treatment from the coal-tar pitch is stronger due to the catalysis of the mesophase. The mesophase therefore has catalytic effect that accelerate the rate of reaction. In accordance with the present invention, large quantity of mesophase is used in substitute for conventional SiO.sub.2 to be reacted with silica gel. With the catalytic activity of the mesophase, the mixture additivity and high-purity of the silica gel, silicon carbide can be produced by using lower industrial temperature that saves precious energy.